SKIN. 295 



ther owes its insolubility, and its power of resisting putrefaction. 

 The subject engaged the attention of Davy, who examined it 

 with his usual ingenuity, and added several important facts to 

 our former knowledge. 



When skins are to be tanned, the first step of the process is to 

 deprive them of their hair and cuticle. This is either done by 

 steeping them in water till they begin to putrefy, or by steeping 

 them in lime and water. The lime seems to combine with the 

 cuticle, and to render it brittle and easily detachable from the 

 hide. It produces the same effect upon the hair and the matter 

 at its root. * When the hides have been steeped for a sufficient 

 time, they are taken out, the hair, cuticle, &c. scraped off, and 

 then they are washed in water. 



After this preliminary process, the skins are subjected to diffe- 

 rent treatment, according to the kind of leather which is to be 

 made. 



The large and thick hides are introduced for a short time into 

 a strong infusion of bark. They are then said to be coloured. 

 After this they are put into water slightly impregnated with sul- 

 phuric acid, or with the acid evolved during the fermentation of 

 barley and rye. This renders them harder and denser than they 

 were before, and fits them for forming sole leather. Davy thinks? 

 that, by this process, a triple compound is formed of the skin, tan, 

 and acid, f 



The light skins of cows, those of calves, and all small skins, 

 are steeped for some days in a lixivium made by the infusion of 

 pigeon's dung in water. This lixivium is called the Drainer. By 

 this process they are rendered thinner and softer, and more pro- 

 per for making flexible leather. Davy considers the effect of this 

 lixivium to depend upon the fermentation which it undergoes ; for 

 dung that has undergone fermentation does not answer the pur- 

 pose | 



After these preliminary processes, the skins are exposed to the 

 action of the infusion of bark till they are converted into leather. 



The infusion of oak bark contains two ingredients, namely, 

 tannin and an extractive. The first is more soluble than the se- 

 cond. Hence, in saturated infusions, there is a much greater 

 proportion of tannin than of extractive ; whereas in weak infu- 



* Davy, Journal of the Royal Instit. ii. 30. f Ibid. p. 31. 



| Ibid. p. 32. 



